This invention relates to a meter for measuring fluid flow by detecting Karman vortices, and particularly to an improved vortex flowmeter that is substantially uneffected by external vibrations.
It is well know that placing an elongated obstruction transverse to the direction of fluid flow within a conduit will result in the generation of vortices in the wake of the obstruction. The vortices are induced by and shed alternately from opposite sides of the obstruction, which is called a bluff body. This has been referred to as the Karman effect. The frequency of the vortex shedding is proportional to the width of the bluff body and the velocity of flow, so that detecting the frequency can generate signals indicative of fluid flow velocity. The shedding frequency is well known and is described by the equation f=Ns V/D, where Ns is the Strouhal number, V is the fluid velocity past the bluff body, and D is the diameter or width of the bluff body.
The vortices are generated in pairs, often referred to as two rows, and are disposed on either side of the longitudinal axis of the bluff body. The rotational direction of the individual vortices is such that each reinforces the other and combines with the other. As the vortices proceed away from the bluff body, the result is loss of individual character for each vortex and the creation of sinuous-like fluid motion transverse to the direction of the velocity of the fluid. In effect, the vortices form a standing transverse wave beyond the bluff body and the wavelength is I=V/f=D/Ns. The sinuous-like wave is persistent, with normally expected dissipation, unless disrupted by some mechanical means. For example, a physical member disposed longitudinally in the stream will substantially dissipate the vortex street formed by the two rows if the length of the member is sufficient. In general, the strength of the vortices increases with increased velocity and with increased fluid density in the relationship of .rho.V.sup.2.
A variety of means for detecting the vortices have been proposed, including the use of acoustic detection (U.S. Pat. No. 3,886,794 issued Jun. 3, 1975 to McShane), hot wires (U.S. Pat. No. 4,275,602 issued Jun. 30, 1981 to Fujishiro, et al), and a physical member disposed downstream of the obstruction and subject to deflection as the alternating vortices pass by. In this latter approach, the physical member often takes the form of a wing and the wing may either be pivotably mounted in the conduit (U.S. Pat. No. 3,116,629 issued Jan. 7, 1964 to Bird and U.S. Pat. No. 4,181,020 issued Jan. 1, 1980 to Herzl) or the wing may be fixed to the conduit (U.S. Pat. No. 4,699,012 issued Oct. 13, 1987 to Lew, et al).
A major shortcoming of flowmeters designed to detect vortex shedding arises from external forces and accelerations in the attached piping systems. Because the physical detection member of a vortex flowmeter is designed to be deflected by the vortices, any other forces which would cause deflection, such as external vibration of the conduit, will be detected as well. This is especially true at lower fluid velocities when the vortex strength is diminished. The external forces (the "noise") thereby adversely affect the accuracy of the signal generated by the physical member. Although many compensating methods have been employed to minimize the noise effect, each has the potential for only small or inadequate reduction of the influences of noise. The present application is directed to an improved vortex flowmeter in which the effects of external noise are substantially eliminated.